US 20060018413 A1 Abstract A system that facilitates obtaining a coarse estimation of a boundary of symbol with respect to time comprises a peak detector that detects a peak energy of an energy distribution output by a correlator, and an estimating component that adaptively estimates a boundary of the symbol based as a function of the detected peak energy. A parameter defined as a function of the magnitude to create a threshold value, the estimate obtained as a function of a comparison of the threshold with the energy distribution.
Claims(79) 1. A method for generating an estimate for a location of a boundary of a symbol with respect to time, comprising:
receiving a symbol; performing a correlation with respect to the symbol; determining a peak energy with respect to the correlation; determining a magnitude value with respect to the located peak energy; and determining the estimate as a function of the magnitude of the peak energy. 2. The method of 3. The method of 4. The method of comparing the threshold with the correlation; determining an instance in time that the threshold is substantially similar to an energy within the correlation; and utilizing the instance in time as the coarse timing estimate. 5. The method of 6. The method of 7. The method of 8. The method of 9. The method of 10. The method of 11. The method of 12. The method of 13. The method of 14. A subscriber station performing the method of 15. The method of where S
_{n }is a correlation metric, {r_{n}} is a received base sample sequence sampled at a Nyquist rate, N is a total number of sub-carriers, and m is a length of a cyclic prefix in number of samples. 16. The method of _{n:n} _{ 0 } _{<n<n} _{ 0 } _{+N+m}|S_{n}|, where {circumflex over (n)} is a location of a sample in time that resides at the peak. 17. The method of 18. The method of 19. The method of 20. The method of 21. A system that facilitates obtaining a coarse estimation of a boundary of a symbol with respect to time, comprising:
a peak detector that detects a peak energy of an energy distribution output by a correlator; and an estimating component that adaptively estimates a boundary of the symbol as a function of a magnitude of the detected peak energy. 22. The system of 23. The system of 24. The system of 25. The system of 26. The system of 27. The system of 28. The system of 29. The system of 30. The system of 31. The system of 32. The system of 33. The system of 34. The system of 35. A subscriber station comprising the system of 36. The system of wherein,
S
_{n }is a correlation metric, {r
_{n}} is a received base sample sequence sampled at a Nyquist rate, N is a total number of sub-carriers, and
m is a length of a cyclic prefix in number of samples.
37. The system of {circumflex over (n)}=arg _{n:n} _{ 0 } _{<n<n} _{ 0 } _{+N+m} |S _{n}|, wherein {circumflex over (n)} is a location of a sample in time that resides at the peak energy. 38. The system of 39. The system of 40. The system of 41. The system of 42. The system of 43. The system of 44. A coarse timing estimation system, comprising:
a correlator; and an estimating component that generates a coarse timing estimate as a function of a threshold that is a function of attributes of an output from the correlator. 45. The system of 46. The system of 47. The system of 48. The system of 49. The system of 50. The system of 51. A system for coarsely estimating a boundary of a symbol in time, comprising:
means for determining a correlation between one or more of samples within a symbol and samples within a plurality of symbols; and means for coarsely estimating the boundary of the symbol in time as a function of attributes of a peak energy of the correlation. 52. The system of 53. The system of 54. The system of 55. The system of 56. The system of means for determining a threshold as a function of the peak energy level; means for comparing the threshold with the correlation; and means for determining an instance in time that the threshold is substantially similar to an energy within the correlation. 57. The system of 58. The system of 58. The system of 59. The system of 60. The system of 61. The system of 62. The system of 63. The system of 64. The system of wherein
S
_{n }is a correlation metric, {r
_{n}} is a received base sample sequence sampled at a Nyquist rate, N is a total number of sub-carriers, and
m is a length of a cyclic prefix in number of samples.
65. The system of {circumflex over (n)}=arg _{n:n} _{ 0 } _{<n<n} _{ 0 } _{+N+m} |S _{n}|, wherein {circumflex over (n)} is a location of a sample in time that resides at the peak. 66. The system of 67. The system of 68. The system of 69. The system of 70. The system of 71. A computer-readable medium having computer-executable instructions for:
receiving an energy distribution representative of a correlation with respect to one or more wireless symbols; and determining a threshold as a function of the energy distribution, a coarse estimate of a boundary of the symbol in time obtained as a function of the threshold. 72. The computer-readable medium of determining a peak energy of the energy distribution; and determining the threshold as a function of a magnitude of the detected peak. 73. The computer-readable medium of defining a parameter; and multiplying the parameter with the magnitude of the peak to generate the threshold. 74. The computer-readable medium of 75. The computer-readable medium of estimating channel noise; and defining the allowable window of time as a function of the estimated channel noise. 76. The computer-readable medium of 77. The computer-readable medium of 78. A microprocessor that executes instructions for determining a boundary of a wireless symbol in time, comprising:
performing a correlation of samples within one or more symbols to determine an energy distribution; and determining a coarse estimate of the boundary as a function of an energy level within the energy distribution. Description This application claims the benefit of U.S. Provisional Application Ser. No. 60/589,898 filed on Jul. 20, 2004, and ACQUISITION FOR MEDIAFLO-PERFORMANCE AND COMPLEXITY ANALYSIS, the entirety of which is incorporated herein by reference. I. Field The following description relates generally to wireless communications, and more particularly to generating a coarse estimate of a symbol boundary with respect to time. II. Background In the not too distant past mobile communication devices in general, and mobile telephones in particular, were luxury items only affordable to those with substantial income. Further, these mobile telephones were of substantial size, rendering them inconvenient for extended portability. For example, in contrast to today's mobile telephones (and other mobile communication devices), mobile telephones of the recent past could not be placed into a user's pocket or handbag without causing such user extreme discomfort. In addition to deficiencies associated with mobile telephones, wireless communications networks that provided services for such telephones were unreliable, covered insufficient geographical areas, were associated with inadequate bandwidth, and various other deficiencies. In contrast to the above-described mobile telephones, mobile telephones and other devices that utilize wireless networks are now commonplace. Today's mobile telephones are extremely portable and inexpensive. For example, a typical modern mobile telephone can easily be placed in a handbag without a carrier thereof noticing existence of the telephone. Furthermore, wireless service providers often offer sophisticated mobile telephones at little to no cost to persons who subscribe to their wireless service. Numerous towers that transmit and/or relay wireless communications have been constructed over the last several years, thus providing wireless coverage to significant portions of the United States (as well as several other countries). Accordingly, millions (if not billions) of individuals own and utilize mobile telephones. The aforementioned technological advancements are not limited solely to mobile telephones, as data other than voice data can be received and transmitted by devices equipped with wireless communication hardware and software. For instance, several major metropolitan areas have implemented or are planning to implement citywide wireless networks, thereby enabling devices with wireless capabilities to access a network (e.g., the Internet) and interact with data resident upon such network. Moreover, data can be exchanged between two or more devices by way of a wireless network. Given expected continuing advancement in technology, a number of users, devices, and data types exchanged wirelessly can be expected to continuously increase at a rapid rate. Communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may be time, frequency, and/or code division multiple-access systems capable of supporting communication with multiple users simultaneously by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Multiple-Carrier CDMA (MC-CDMA), Wideband CDMA (W-CDMA), High-Speed Downlink Packet Access (HSDPA), Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems. Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) are exemplary protocols that are currently utilized in wireless environments to transmit and receive data. OFDM modulates digital information onto an analog carrier electromagnetic signal, and is utilized in an IEEE 802.11a/g WLAN standard, 802.16, and 802.20. An OFDM base band signal (e.g., a subband) is a sum of a number of orthogonal sub-carriers, where each sub-carrier is independently modulated by its own data. Benefits of OFDM over other conventional wireless communication protocols include ease of filtering noise, ability to vary upstream and downstream speeds (which can be accomplished by way of allocating more or fewer carriers for each purpose), ability to mitigate effects of frequency-selective fading, etc. To effectively employ OFDM as a communications protocol, a boundary between symbols in an OFDM environment often needs to be determined. Such symbols include a plurality of samples as well as a cyclic prefix. The cyclic prefix, for example, can be located at a portion of a symbol first in time, and can include samples that exist within the symbol last in time. Thus, a boundary between symbols that include cyclic prefixes can be determined by locating a cyclic prefix within wireless symbols. A correlating component (e.g., a cross-correlator, an autocorrelator, a delay correlator, . . . ) correlates the cyclic prefix with samples within the symbol substantially similar thereto and determines a correlation in energy therebetween. A peak energy level output by the correlating component is indicative of a boundary of a symbol that can be employed in a wireless environment, and thereafter a fast Fourier transform can be applied to samples in a symbol delivered next in time. If multi-path effects were not an issue and no noise existed upon such channel, the peak energy output by the correlating component could be utilized to precisely determine a boundary between symbols adjacent in time. Channels, however, are frequently associated with various noise, thus rendering it more difficult to determine location of a peak energy level output by a correlating component. Further, often channels are subject to a multi-path effect, wherein disparate portions of a symbol are delivered over different physical paths (or substantially similar portions of a signal are delivered over disparate physical paths), which can cause delay with respect to a receiver obtaining a plurality of samples. Thus, output of a correlator can produce a heightened flat energy level that does not include a peak corresponding to a boundary between symbols in a wireless network (e.g., OFDM, OFDMA, . . . ). Moreover, when noise accumulates on a channel, accurately determining a boundary between symbols based upon a peak can be difficult. In particular, if there is substantial disparity with respect to location in time of an energy peak output by the correlating component and location of a boundary, errors can result, thereby compromising network performance. In an attempt to alleviate such errors, conventional systems utilize a pre-defined time measurement and utilize such measurement to estimate the aforementioned boundary. In particular, a coarse estimate of a boundary between symbols is obtained by traversing backwards in time from an occurrence of a peak energy level (as output by the correlating component) the pre-defined amount of time. Such a methodology is adequate when a channel is not subject to noise and/or severe multi-path effects. During instances that a channel is associated with substantial noise, this approach can result in error that renders adequate obtainment of a coarse timing estimate between symbols problematic. In view of at least the above, there exists a need in the art for a system and/or methodology obtaining an improved coarse estimation of a boundary between wireless symbols with respect to time. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. One or more embodiments include a plurality of systems/methodologies for obtaining improved coarse timing estimates with respect to symbols in wireless networking environments (e.g., OFDM, OFDMA, . . . ). To facilitate optimal communications, a receiver should have an ability to precisely determine a boundary between disparate symbols. If there is error in such timing estimate, then substantial demodulation errors can result, thus reducing performance and increasing user frustration. One embodiment utilizes a cyclic prefix correlator to generate a correlation metric with respect to a symbol in a wireless environment. In a noiseless channel, a peak of the correlation metric (e.g., a peak of an energy distribution indicative of similarity between samples) will directly correspond to a boundary of the symbol. In a channel subject to multi-path effects, however, due to timing delays and noise, a peak of such correlation metric (e.g., a point in time that a greatest energy level exists) will not precisely correspond to a boundary of the symbol. To obtain an improved coarse estimate of the boundary of the symbol, an adaptive technique is employed. More specifically, rather than utilizing a fixed time and performing a fixed “back off” from the peak (and labeling a result of such back off as a coarse estimate), an adaptive “back off” technique is employed. For example, a magnitude of the correlation metric at the peak can be stored and used in connection with generating a coarse estimate. In other words, a coarse estimation can be determined as a function of a determined peak magnitude. In one example, a parameter (between 0.5 and 1) can be defined and multiplied with the stored magnitude of the peak. A result of such multiplication can be utilized as a threshold, and the threshold is adaptive as peak magnitudes can alter given disparate channel conditions, disparate symbols, etc. The threshold is compared with correlator metrics, and a coarse timing estimate with respect to a boundary of the OFDM symbol is acquired as a function of the comparison. For instance, an energy distribution output by the cyclic prefix correlator can be compared with the determined threshold. A first point in time prior to occurrence of the peak energy level that the threshold is substantially similar to an energy output by the correlator can be defined as a coarse timing estimate with respect to a boundary of the symbol, wherein the symbol can be an OFDM symbol, an OFDMA symbol, or any other suitable symbol that can be employed in a wireless environment. In accordance with another exemplary embodiment, a time window can be defined, wherein a coarse timing estimate is required to be within the time window. For instance, an energy distribution output by cyclic prefix correlator can be compared with a generated threshold as a function of a magnitude of a peak of the energy distribution. A time corresponding to where the threshold is first substantially similar to the energy distribution prior to occurrence of the peak, however, can lie outside the defined time window. Upon such an occurrence, a parameter utilized in connection with generating the threshold can be altered. For example, an algorithm can be utilized to alter the parameter, and thus alter the threshold. Moreover, both the parameter and the time window can be defined based upon channel condition estimates, previous performance, and the like. For example, the time window can be expanded during instances of high data volume to ensure that the coarse timing estimate is located quickly. Similarly, the parameter can be defined to be a greater value during instances of increased noise, and defined to be a lesser value during instances of greater path delay. In another aspect, a method for generating an estimate for a location of a boundary of a symbol with respect to time is described herein. The method comprises receiving a symbol and performing a correlation with respect to the symbol. Thereafter, a peak energy with respect to the correlation is determined and a magnitude value with respect to such located peak energy is determined. The estimate of the boundary is determined as a function of the magnitude of the peak energy. In yet another aspect, a system that facilitates obtaining a coarse estimation of a boundary of a symbol with respect to time is described herein. The system comprises a peak detector that detects a peak energy of an energy distribution output by a correlator. An estimating component adaptively estimates a boundary of the symbol as a function of a magnitude of the detected peak energy. To the accomplishment of the foregoing and related ends, the one or more embodiments comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the one or more embodiments. These aspects are indicative, however, of but a few of the various ways in which the principles of various embodiments may be employed and the described embodiments are intended to include all such aspects and their equivalents. Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiment(s) may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments. As used in this application, the terms “component,” “handler,” “model,” “system,” and the like are intended to refer to a computer-related entity, either hardware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer. One or more components may reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers. Also, these components can execute from various computer readable media having various data structures stored thereon. The components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets (e.g., data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal). In accordance with one or more embodiments and corresponding disclosure thereof, various aspects are described in connection with a subscriber station. A subscriber station can also be called a system, a subscriber unit, mobile station, mobile, remote station, access point, base station, remote terminal, access terminal, user terminal, user agent, or user equipment. A subscriber station may be a cellular telephone, a cordless telephone, a Session Initiation Protocol (SIP) phone, a wireless local loop (WLL) station, a personal digital assistant (PDA), a handheld device having wireless connection capability, or other processing device connected to a wireless modem. Communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may be time, frequency, and/or code division multiple-access systems capable of supporting communication with multiple users simultaneously by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Multiple-Carrier CDMA (MC-CDMA), Wideband CDMA (W-CDMA), High-Speed Downlink Packet Access (HSDPA), Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems. Referring now to the drawings, After processing the symbols, the correlator outputs the energy distribution More specifically, a peak of the energy distribution The system The system The system Turning now to Output of the correlator In one exemplary embodiment, an estimating component Such instance in time can be employed as a coarse estimate unless the instance occurs outside a defined time window Now referring to A peak detector The estimating component The estimating component Now turning to The parameter generating component As the machine learning component Referring to Referring now solely to At At Turning now to At Referring now to At Now turning to Referring now to Now turning to The systems and methods of one or more embodiments improve coarse estimations by adaptively “backing off” from a time associated with the peak energy Referring collectively to As described previously, OFDM symbol timing estimates can be improved by utilizing the estimate described above with respect to It can be determined that mean timing error can be altered by way of modifying a value of t. While modifying conventional systems and methodologies by utilizing an adaptive threshold (a magnitude of a cyclic prefix correlation multiplied by a parameter), timing error standard deviation remains too large to be acceptable as a fine timing result. A performance parameter directly related to modem performance is a percentage of energy captured within a FFT window. A disparate scheme that can be employed to obtain a coarse timing estimate by utilizing a fixed threshold rather than a fixed back off. A coarse timing estimate is declared when |S Reviewing the graph Referring now to As integer frequency and frame acquisition occurs prior to channel estimates are available for post-FFT fine symbol timing synchronization, integer frequency and frame synchronization should be acquired with pre-FFT coarse timing described above. Performance of timing and fractional frequency acquisition is shown in A time-frequency acquisition sequence follows utilizing pre-FFT and post-FFT refinements described above. At a first act, a maximum |S At a third act, a maximum |S At a fifth act, integer frequency offsets can be estimated utilizing FFT outputs from two consecutive OFDM symbols. At a sixth act, integer frequency offsets can again be estimated from two consecutive OFDM symbols. If this estimate is substantially similar to an estimate obtained in the fifth act, integer frequency can be declared as acquired. Integer frequency correction can be applied pre-FFT for subsequent OFDM symbols, initiating from a beginning of a new symbol. If the estimate of the sixth act is not substantially similar to the estimate of the fifth act, the fifth act can be revisited. At a seventh act, frame synchronization can be acquired. At an eight act, carrier channel estimates can be obtained from staggered pilots from two consecutive OFDM symbols. Thereafter, a 2m point IFFT can be undertaken, and symbol timing can be estimated and applied to a pre-FFT window for subsequent OFDM symbols. Thereafter, OFDM symbol timing can be declared as acquired. On a subsequent wake-up, channel delay profile may have significantly been altered, and a carrier frequency may have substantially drifted (but drifted less than ±Δf/2 where Δf is an inter-carrier spacing). Accordingly, OFDM symbol timing and fractional frequency should be re-acquired. As the pre-FFT timing is coarse, it may be beneficial to not utilize such timing—timing correction can be completed by employing a post-FFT technique. At a first act, cyclic prefix correlation can be performed on non-decimated samples, and fractional frequency offset can be estimated. For subsequent OFDM symbols apply such frequency correction at a beginning of a new symbol. At act Referring now to A transmitter unit The processor The uplink signal from the terminal For multiple-access OFDM systems (e.g., an orthogonal frequency division multiples access (OFDMA) systems), multiple terminals can transmit concurrently on the uplink. For OFDMA and similar systems, pilot sub-carriers can be shared amongst disparate terminals. This pilot sub-carrier structure can be desirable to obtain frequency diversity for differing terminals. The channel estimation techniques described herein can be implemented through various means/devices. For example, hardware, software, or a combination thereof can be employed to obtain a channel estimation in accordance with one or more aforementioned embodiments. For example, the processing units employed for channel estimation purposes can be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, and/or any other suitable device/unit or a combination thereof. With respect to software, a channel estimation in accordance with one or more previously described embodiments can be obtained at least in part through use of modules (e.g., procedures, functions, . . . ) that perform one or more functions described herein. Software can be stored in memory, such as the memory units What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim. Referenced by
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